Just make sure you clarify with them exactly how many cylinders that gets you on the boats. When we booked last November, the advertisement was very clear about the boat dives - 5 x 2 tank dives = 10 cylinders. When the vouchers arrived 5 days before traveling in February, the voucher stated 6 x 1 tank dives = 6 cylinders. In the end because we were in a group and so had a boat to ourselves we rearranged this into 3 x 2 tank dives = 6 cylinders. We didn't kick up too much of a fuss because in Bonaire we just did more shore dives, which were more enjoyable anyway, but if you're counting on only diving from a boat it would make a difference.
No matter where you book, or where you travel, it pays to keep copies of all contacts, vouchers, etc, so you can show staff what you were promised if there is a problem. Same thing with airlines,etc. Copies of all contact document and emails have become a part of my travel documents. Lots easier to resolve a misunderstanding when you have hard copies, or access to email copies that you can retrieve.